How does novelty search work?
As explained in an answer to this AI SE question, GAs are 'satisficers' rather than 'optimizers' and tend not to explore 'outlying' regions of the search space. Rather, the population tends to cluster in regions that are 'fairly good' according to the fitness function.

In contrast, I believe the thinking is that novelty affords a kind of dynamic fitness, tending to push the population away from previously discovered areas.